Photoelectric device efficiency depends on many factors, both extrinsic and intrinsic. Of the two classes, intrinsic factors set a limit on the maximum efficiency that a photoelectric device can achieve. Dominant intrinsic factors include losses due to (i) lack of absorption of photons, (ii) exciton relaxation and (iii) radiative recombination. The first loss results from the failure of a semiconductor to absorb photons with energy less than the energy band gap of the semiconductor. The second loss takes place when photons with energy greater than the energy band gap of the semiconductor generates electrons and holes which immediately lose to heat almost all energy in excess of the energy band gap of the semiconductor. And the third loss is due to the recombination of electron-hole pairs generating light.
Reducing these intrinsic losses is a goal in photoelectric device development, particularly for devices used over a broad spectrum of wavelengths. For instance, solar radiation ranges from 100 nm to 14 μm, with the visible light ranging from 400 nm to 700 nm. Assuming all extrinsic losses are eliminated, the maximum efficiency is about 31 percent for an ideal solar cell made from a single semiconducting material with an optimal band gap of about 1.35 eV (See, Journal of Applied Physics 51, 4494 (1980)). That is, 69 percent of solar energy is not cultivated due to the intrinsic losses.
One strategy for improving device efficiency is to use multijunction photoelectric devices having materials with multiple band gaps. Some existing multijunction photovoltaic devices are built from group III-V semiconductors (See, Energy and Environmental Science, 2, 174-192 (2009)). A typical structure of a multijunction photovoltaic device comprises a number of n-p (or p-n) junctions made from different semiconducting materials stacked on top of each other. Each junction has an energy band gap higher than the junction below it; and an interface is disposed in between the stacked junctions. FIG. 1 shows a generalized three junction structure of a series-connected, monolithically-grown, GaInP/GaInAs/Ge stack (See, U.S. Pat. No. 6,660,928 to M. O. Patton).
Several issues need to be addressed in order to fabricate a multiple band gap device, such as a three junction photovoltaic device depicted in FIG. 1. First, semiconducting materials for different layers typically need to have a matched lattice constant in order to epitaxially grow on a substrate and to form p-n or n-p junctions. Second, to facilitate improved efficiency, the interfaces between two n-p (or p-n) junction layers typically need to have a low resistance to enable the generated current to flow from one junction to the next. Accordingly, in a monolithic structure, low resistive tunnel junctions have been used to minimize the blockage of current flow. Third, current density generated in each layer typically needs to be roughly the same so that the lowest photogenerated current density does not limit the current flowing through the multijunction device.
Such requirements impose technical challenges in the fabrication of semiconducting materials and multijunction devices. It is difficult to construct three different semiconducting materials having desired band gaps, and at the same time, meeting other design goals such as a matched lattice constant. For example, the three junctions in the GaInP/GaAs/Ge system are limited to the respective band gaps of 1.8, 1.4, and 0.67 eV, respectively (See, Energy and Environmental Science 2, 174-192 (2009)). This leads to a non-ideal combination of band gaps, and consequently lower device efficiency.
Efforts have been directed to modifying the composition of semiconducting materials by adjusting band gaps and balancing the current. Examples can be found in U.S. Pat. No. 6,340,788 to R. R. King et al., which discloses several three junction cells including (i) Ga0.52In0.48 P, GaAs, and Si0.17Ge0.83 devices with respective band gaps of 1.89, 1.42, and 0.92 eV, (ii) Ga0.55In0.45P, GaP0.07As0.93 and Si devices with respective band gaps of 1.94, 1.51, and 1.12 eV, (iii) Ga0.52In0.48P, GaAs and Si devices with respective band gaps of 1.89, 1.42, and 1.12 eV, and (iv) Ga0.52In0.48P, GaxIn1-xPyAs1-y, and Si devices with respective band gaps of 1.89, 1.50 and 1.12 eV.
Meanwhile, efforts have been directed to employing more p-n (or n-p) layers to improve the efficiency. For example, U.S. Pat. No. 6,340,788 to et al., discloses a number of four-junction solar cells while U.S. Pat. No. 6,316,715 to King et al. discloses solar cells having three, four or five-junctions.
Although modification of the composition of semiconducting materials may improve device efficiency, maximum efficiencies have not yet been achieved to date because of practical obstacles in building such devices. Such obstacles include facilitating epitaxial growth of desired compositions due to miscibility gap of multiple layers on top of each other while satisfying interlayer lattice matching constraints and the need to balance the amount of current generated in each junction in the multijunction stack. That is, composition tuning, lattice constant matching, and current balancing complicate the multijunction fabrication process and increase the cost of production of such devices.
Given the above background, there is a need in the art for improved devices that are easier to fabricate and/or have improved efficiency or other improved characteristics.